Backlight control device and method for controlling a driving current of an led

ABSTRACT

A backlight control device for controlling a driving current of an LED is disclosed. By controlling current outputs from current sources of a plurality of current output units, a display will be able to generate desirable backlight. Then by adjusting currents output by the plurality of current output units, brightness of a plurality of pixels can be dynamically adjusted. The brightness of pixels with higher gray levels can be increased while the brightness of pixels with lower gray levels can be decreased, thereby improving the contrast of image and saving power consumption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight module of a thin filmtransistor liquid crystal display, and more particularly, to a backlightcontrol device for controlling a driving current of an LED.

2. Description of the Prior Art

Ordinarily, the current control integrated circuit of a light-emittingdiode (LED) uses fixed resistance to control the driving current of thebacklight module to control the brightness of the backlight module. Thedisadvantage is that once the resistance value of the bias resistor isestablished, the driving current of backlight module cannot be adjusted,and a long-term use of circuit components can cause an unexpectedvariation in the driving current of backlight module. Further, thedriving current in certain types of LED, such as Organic LED (OLED), maybe changed due to a change in the operating temperature of the currentregulator. As a result, the brightness of the LED in the backlightmodule may be adversely affected. Therefore, a method and an apparatuscapable of controlling the driving current for backlight modules in LCDsystems are needed.

FIG. 1 illustrates a prior art programmable control method to regulatethe brightness of LEDs in a backlight module 100. Microcontrollers arean example where users can input the desired current value or voltagevalue to an LED controller 103 through a user interface 101 of themicrocontroller 102. According to the current value or voltage valuestored in registers, the desired current value or voltage value isgenerated and provided to the current output unit, and the outputcurrent 104 generated by the current output unit is used to regulate thebrightness of an LED 105.

However, the method described above uses programming interfaces toregulate the brightness of LEDs, so that the backlight can not regulatethe brightness in displaying video. Therefore, the related art can notdynamically reduce the brightness of backlight when the lower brightnessis desired. Further, the related art can not dynamically enhance thebrightness of backlight when the higher brightness is desired. Therelated art uses brightness predetermined and inputted by a user, andthe backlight module can not dynamically regulate the brightness lateron. As a result, it causes low quality in the brightness contrast ofimages.

SUMMARY OF THE INVENTION

An embodiment of the present invention discloses a backlight controldevice for controlling a driving current of a light-emitting diode. Thebacklight control device comprises at least a pulse width modulation(PWM) control unit, at least a current output unit coupled to the PWMcontrol unit, and a shift register coupled to the PWM control unit forstoring control signals for controlling the PWM control unit.

Another embodiment of the present invention discloses a method forcontrolling a driving current of an LED. The method comprises outputtinga predetermined current to cause the LED to emit uniform light, andafter outputting the predetermined current, outputting a voltageproportional to brightness of an image to be displayed.

These and other objectives of the present invention will no doubt becomeapparent to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art programmable control method to regulatethe brightness of LEDs in a backlight module.

FIG. 2 illustrates a backlight control device according to a firstembodiment of the present invention.

FIG. 3 illustrates of a backlight control device according to a secondembodiment of the present invention.

FIG. 4 illustrates a backlight control device according to a thirdembodiment of the present invention.

FIG. 5 illustrates a backlight control device according to a fourthembodiment of the present invention.

FIG. 6 illustrates a backlight control device according to a fifthembodiment of the present invention.

FIG. 7 illustrates a basic operating timing diagram according to thepresent invention.

FIG. 8 is a PWM timing diagram of all channels when the backlightcontrol device is in the dynamic PWM mode.

FIG. 9 shows an embodiment of controlling red, green and blue primarycolors according to the present invention.

DETAILED DESCRIPTION

FIG. 2 illustrates a backlight control device 200 according to a firstembodiment of the present invention. The backlight control device 200comprises a plurality of PWM control units 202 each including a PWMregister 212 and a control unit 214, a plurality of current output units204 coupled to the PWM control units 202, and a shift register 206coupled to the PWM registers 212 of the PWM control units 202. The shiftregister 206 stores brightness control signals for the PWM control units202.

Each current output unit 204 comprises a current source 208, and an NMOS210. The current source 208 is coupled to a low voltage terminal such asground. The source of NMOS 210 is coupled to the current source 208, andthe gate is coupled to the PWM control unit 202. Each PWM control unit202 comprises a PWM register 212 coupled to the shift register 206 forstoring a brightness control signal. The control unit 214 outputs acorresponding PWM signal according to the brightness control signalstored in the PWM register 212. The PWM signal is a periodic signalwhich in some periods is at a high voltage level and in other periods isat a low voltage level. The periodic signal is used to regulate thecurrent by opening and closing the gate of the NMOS 210. When the PWMsignal is at the high voltage level, the gate of the NMOS 210 is opened.Thus a higher percentage of high voltage in the PWM signal correspondsto a greater current output by the current output unit 204, whichcorresponds to higher backlight brightness. On the other hand, a lowerpercentage of high voltage in the PWM signal corresponds to a lowercurrent output by the current output unit 204, which corresponds tolower backlight brightness. Therefore the PWM signal outputted by thecontrol unit 214 can dynamically regulate the backlight brightnessaccording to the display signal. The input of the control unit 214 iscoupled to the PWM register 212. The output of the control unit 214 iscoupled to the gate of the NMOS 210 which is used to regulate the outputcurrent of the current output unit 204 through opening and closing thegate of the NMOS 210.

In this embodiment, the shift register 206 is coupled to a plurality ofPWM control units 202, and the memory size of the shift register 206 isnot greater than the total memory size of all PWM registers 212.Therefore, a brightness control signal of a first channel is stored inthe shift register 206, and then it is transferred to a PWM register 212of the first channel. Afterwards a brightness control signal of a secondchannel is stored in the shift register 206, and then it is transferredto a PWM register 212 of the second channel. The shift register 206transfers brightness control signals to PWM registers 212 of differentchannels sequentially so as to control the output currents of differentcurrent output units 204.

FIG. 3 illustrates of a backlight control device 300 according to asecond embodiment of the present invention. The difference between thebacklight control device 300 and backlight control device 200 is thatthe current output unit 304 of the backlight control device 300comprises a current source 308 and a PMOS 310. The current source 308 iscoupled between a low voltage terminal such as ground and the drain ofPMOS 310. Like the backlight control device 200, the backlight controldevice 300 regulates the output current of the current output unit 304through opening and closing of the gate of PMOS 310.

The PWM signal outputted from the control unit 314 is a periodic signalwhich in some periods is at a high voltage level and in other periods isat a low voltage level. The periodic signal is used to regulate thecurrent by opening and closing the gate of the PMOS 310. When the PWMsignal is at the low voltage level, the gate of the PMOS 310 is opened.Thus a higher percentage of low voltage in the PWM signal corresponds toa greater current output by the current output unit 304, whichcorresponds to higher backlight brightness. On the other hand, a lowerpercentage of low voltage in the PWM signal corresponds to a lowercurrent output by the current output unit 304, which corresponds tolower backlight brightness. Therefore the PWM signal outputted by thecontrol unit 314 can dynamically regulate the backlight brightnessaccording to the display signal. The input of the control unit 314 iscoupled to the PWM register 312. The output of the control unit 314 iscoupled to the gate of the PMOS 310 which is used to regulate the outputcurrent of the current output unit 304 through opening and closing thegate of the PMOS 310.

FIG. 4 illustrates a backlight control device 400 according to a thirdembodiment of the present invention. The difference between thebacklight control device 400 and backlight control device 200 is thatthe backlight control device 400 comprises a plurality of shiftregisters 406, and they are coupled to a plurality of PWM control unit202.

The first shift register is coupled to the PWM control unit of the firstchannel. The second shift register is coupled to the PWM control unit ofthe second channel and so on. Each shift register 406 is coupled to thePWM control unit 202 of a corresponding channel. Compared to the firstembodiment shown in FIG. 2, the one-to-one coupling in FIG. 4 is a lotsimpler than the one-to-many coupling in FIG. 2.

FIG. 5 illustrates a backlight control device 500 according to a fourthembodiment of the present invention. The backlight control device 500comprises a plurality of PWM control units 503, a plurality of currentcontrol units 504, a plurality of current output units 505, a selector502, and a shift register 501 for storing control signals of the PWMcontrol units or control signals of the current control units 504.

Each current output unit 505 comprises a current source 507 and an NMOS506. The current source 507 is coupled between a low voltage terminaland the source of the NMOS 506. The gate of the NMOS 506 is coupled tothe PWM control unit 503.

Each PWM control unit 503 comprises a PWM register 508 and a controlunit 509. The first input of the PWM register 508 is coupled to theselector 502, the second input of the PWM register 508 is coupled to theshift register 501, and the output of the PWM register 508 is coupled tothe input of a corresponding control unit 509. The output of the controlunit 509 is coupled to the gate of the corresponding NMOS 506.Therefore, The backlight control device 500 regulates the output currentof the current output unit 505 through opening and closing the gate ofNMOS 506.

Each current control unit 504 comprises a register 510 and a controlunit 511. The first input of the register 510 is coupled to the selector502, and the second input of the register 510 is coupled to the shiftregister 501. The output of the register 510 is coupled to the input ofa corresponding control unit 511. The output of the control unit 511 iscoupled to the source of the corresponding NMOS 506, and is used toregulate the voltage of the source so as to regulate the output currentof output current unit 505.

Further, due to the differences among electronic components of abacklight module, or due to a long time use of certain electroniccomponents, the driving current of the backlight module may haveunpredictable changes. The driving current in certain types of LEDs,such as organic LEDs (OLEDs), can change due to a change in theoperating temperature of the current regulator. The current control unit504 allows a user to adjust the luminance of LEDs inside the backlightmodule through the programmable interface. For instance, amicrocontroller can use an I²C programmable interface to input a desiredcurrent or voltage value to the shift register 501, and then the desiredvalue will be transferred to the current control unit 504. According tothe value stored in the register 510, the control unit 511 controls thecurrent output unit 505 to regulate the current output by the currentoutput unit 505 so as to regulate the brightness of LEDs.

In this embodiment, the first output of the shift register 501 iscoupled to a plurality of PWM control units 503 and the second output ofthe shift register 501 is coupled to a plurality of current controlunits 504. Whether the output of the shift register 510 is to be inputto the PWM control units 503 or the current control units 504 depends onthe mode of the selector 502. Therefore the shift register 501 willstore the control signals for every channel and transfer those controlsignals to the PWM control units 503 or the current control units 504 insequence.

FIG. 6 illustrates a backlight control device 600 according to a fifthembodiment of the present invention. The difference between thebacklight control device 600 and backlight control device 500 is thatthe current output unit 605 of the backlight control device 600comprises a current source 607 and a PMOS 606. The current source 607 iscoupled between a low voltage terminal and the drain of PMOS 606. Likethe backlight control device 500, the backlight control device 600regulates the output current of the current output unit 605 throughopening and closing the gate of PMOS 606.

In the embodiment of FIG. 5, by regulating the currents of the currentoutput units 505 from the current control units 504 through theprogrammable interface, a user can regulate the brightness of LEDs inthe backlight module so that uniform brightness of backlight can beobtained when the grey levels of different channels are the same. Theapplication can be used in a PC monitor because most of its frames arestatic, and the distance between a user's eyes and the screen of monitoris relatively short. The user will feel unpleasant if the backlight isnot uniform due to the characteristics of different LED electroniccomponents. The application can also be used in a monitor of the medicalscience field. If the LEDs cannot generate backlight of uniformbrightness, the image displayed on the monitor may be distorted andresult in misjudgments of physicians.

The embodiments in FIG. 2, FIG. 3 and FIG. 4 do not have the currentcontrol units 504 shown in FIG. 5. The reason is that most framesdisplayed on TV are dynamic. Thus the grey levels change from picture topicture. In NTSC system there are 24 frames per second showing on TV, soit is difficult for a human eye to identify if the backlight is uniformor not. Instead the human eye is more sensitive to brightness contrast.Therefore, when displaying a frame or a section with lower brightness,the brightness of backlight of the frame or section is reduced. And whendisplaying a frame or a section with higher brightness, the brightnessof backlight of the frame or section is enhanced. This will result in ahigher visual contrast. Further when using lower brightness fordisplaying a dimmer image, power can be saved due to a lower current.Besides, the TV can be installed with the current control unit 504 tomore accurately reflect the image to be displayed. In this case, the TVcan be switched between the dynamic PWM mode or current mode accordingto the user's need to improve the quality of image display.

In the related art, a desired brightness value is input by a user. Itcan not dynamically regulate the brightness of backlight according thegrey levels of frames to be displayed causing a low quality of imagecontrast. In the present invention, the backlight of uniform brightnesscan be provided and power can be saved by dynamically adjusting thebrightness of backlight. Thus the present invention solves the problemof non-uniform backlight, enhances the brightness contrast, and savespower.

FIG. 7 illustrates a basic operating timing diagram according to thepresent invention. A low voltage signal MODE means that the backlightcontrol device is in the current mode, and a high voltage signal MODEmeans that the backlight control device is in the dynamic PWM mode. Thedata will be latched according to signal CLK when the signal DIO istriggered. When the backlight control device is in the current mode, thelatched data are desired current values. On the other hand, when thebacklight control device is in the dynamic PWM mode, the latched dataare grey levels of images to be displayed. The rising edge of signalMODE signals that the operation of LED current sink IC is changed fromthe current mode to dynamic PWM mode. At this moment, the desiredcurrent values are latched in the register of the current control unit.

In the dynamic PWM mode, when the signal BLANK is at a rising edge, thedata are the grey levels of images to be displayed and will be latchedby the PWM register. When the signal BLANK is at the low voltage, theoutputs of all channels are equal to zero. When the signal BLANK is atthe high voltage, the outputs of all channels are dependent on the greylevels latched by the PWM registers. The next frame appears when thesignal BLANK is at the falling edge. At this time, the control signal ofthe PWM control unit is reset to zero so as to calculate the controlsignals for the next frame.

The signal CLK is the timing signal for the PWM control units, the PWMregisters and registers of current control units.

FIG. 8 is a PWM timing diagram of all channels when the backlightcontrol device is in the dynamic PWM mode. When the signal BLANK is atthe rising edge, the latched data (grey levels of video frames) arestored in the PWM registers of PWM control units. When the signal BLANKis at the falling edge, the control signals of PWM control units arereset to zero.

FIG. 9 shows an embodiment of controlling red, green and blue primarycolors according to the present invention. The red, green and bluebacklight devices can use any of the backlight control devices shown inFIG. 2, 3, 4, 5 or 6 for controlling the red, green and blue backlight.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention.

1. A backlight control device for controlling a driving current of alight-emitting diode, comprising: a pulse width modulation (PWM) controlunit; a current output unit coupled to the PWM control unit; and a shiftregister, coupled to the PWM control unit, for storing control signalsfor controlling the PWM control unit.
 2. The backlight control device ofclaim 1 wherein the current output unit comprises: a current source; andan NMOS having a source coupled to the current source, and a gatecoupled to the PWM control unit.
 3. The backlight control device ofclaim 1 wherein the current output unit comprises: a current source; anda PMOS having a drain coupled to the current source, and a gate coupledto the PWM control unit.
 4. The backlight control device of claim 1wherein the PWM control unit comprises: a register coupled to the shiftregister; and a control unit having an input node coupled to theregister, and an output node coupled to the current output unit.
 5. Abacklight control device for controlling a driving current of alight-emitting diode comprising: a pulse width modulation (PWM) controlunit; a current control unit; a current output unit coupled to the PWMcontrol unit and the current control unit; a selector coupled to the PWMcontrol unit and the current control unit; and a shift register coupledto the PWM control unit and the current control unit for storing controlsignals for controlling the PWM control unit or the current controlunit.
 6. The backlight control device of claim 5 wherein the currentoutput unit comprises: a current source coupled to the current controlunit; and an NMOS having a source coupled to an input node of thecurrent source, and a gate coupled to the PWM control unit.
 7. Thebacklight control device of claim 5 wherein the current output unitcomprises: a current source coupled to the current control unit; and aPMOS having a drain coupled to an input node of the current source, anda gate coupled to the PWM control unit.
 8. The backlight control deviceof claim 5 wherein the PWM control unit comprises: a register having afirst input node coupled to the shift register, a second input nodecoupled to the selector, and an output node; and a control unit havingan input node coupled to the output node of the register, and an outputnode coupled to the current output unit.
 9. The backlight control deviceof claim 5 wherein the current control unit comprises: a register havinga first input node coupled to the shift register, a second input nodecoupled to the selector, and an output node; and a control unit havingan input node coupled to the output node of the register, and an outputnode coupled to the current output unit.
 10. A method for controlling adriving current of a light-emitting diode (LED), comprising: outputtinga predetermined current to cause the LED to emit uniform light; andafter outputting the predetermined current, outputting a voltageproportional to brightness of an image to be displayed.